1. Field of the Invention
The present invention relates to a microphone apparatus suitable for a public-address system and the like, and particularly to a microphone apparatus which can prevent howling.
2. Related Background of the Invention
There occurs howling in a public-address system and the like. The howling occurs because a kind of oscillator is constituted by a serial connection of an acoustic system and an electric system, in such a manner that an amplified acoustic wave emitted from a speaker enters into a microphone and transformed into an audio signal in the microphone, and then the audio signal is amplified to be emitted from the speaker as the amplified acoustic wave. When howling occurs, an unpleasant loud sound is emitted from a speaker.
When howling starts to occur, amplitude of diaphragm in a microphone increases from a small one to a large one. At a timing howling likely starts, the howling tends to stop by taking a measure of adjusting an amplifier gain to reduce a loop gain while the amplitude of the diaphragm is still comparatively small. After the amplitude has become large, however, the howling sometimes can not be suppressed even by a considerable reduction of the amplifier gain. This is because the diaphragm of a microphone is in a persistent oscillation. Measures of preventing howling include various kinds of measures in an acoustic system or an electric system as follows.
Measures in an acoustic system include:
1. changing a position or a direction of a microphone;
2. changing directivity of a microphone from a cardioid pattern to a hyper-cardioid pattern, for example; and
3. changing a position or a direction of a speaker.
Measures in en electric system include:
1. changing phase and amplitude of a audio signal; and
2. removing a specific frequency range with a filter.
The above mentioned measures of preventing howling in a acoustic system, however, are stopgap measures and not fundamental measures, and also take a long time to stop howling, since a position or a direction of a microphone and a speaker or directivity of a microphone is changed through a trial and error process in a situation in which howling is caused by various overlapped conditions.
Also, the above mentioned measures of preventing howling in an electric system have problems that a complicated electric circuit configuration is necessary and also a high fidelity sound of an acoustic wave captured by a microphone can not be reproduced by a speaker, since audio signal thereof is processed.
Technologies disclosed by patent references regarding howling prevention include the followings. One of them is an anti-howl back device which receives a sound input into any one of a plurality of microphones, stores an audio signal thereof into an audio memory via a switching circuit, and reads out the audio signal stored in the audio memory to supply it to each speaker via an amplifier after having finished receiving the sound input into the microphone (refer to patent reference 1). That is, a sound input into each microphone is amplified and reproduced in each speaker after the use of microphone is stopped and thereby howling does not occur.
There is proposed a sound processing apparatus for preventing occurrence of a howling phenomenon, including a transforming means collecting a sound from a sound source and transforming the sound into an electric signal to output the signal, a sound generating means generating a sound corresponding to the electric signal output from the transforming means, and a delaying means delaying the electric signal generated by the transforming means and supplying the delayed signal to the sound generating means, before the sound is generated by the sound generating means (refer to patent reference 2).
Also, there is known a sound collecting apparatus configured so as to: make N channels of pseudo-object sound sources as reference signals; add components from dead angle directions formed by a set of N input channels from M sound collecting means; make the N channel pseudo-object sound sources as input signals; operate N adaptive filters; set obtained N adaptive filter coefficient values to N variable filters; process signals from N sound collecting means among the M sound collecting means; and sum up N outputs thereof (patent reference 3). This reference intends to prevent howling by using a plurality of sound collecting means and by adaptively reducing sensitivities thereof to a speaker largely to attenuate a total sensitivity to the speaker.    Patent reference 1: Japanese Unexamined Patent Application Publication No. 10-200986    Patent reference 2: Japanese Unexamined Patent Application Publication No. 5-14990    Patent Application 3: Japanese Unexamined Patent Application Publication No. 2003-87891